The present invention relates to electrochemical supercapacitors and, in particular, to a supercapacitor where electrical storage occurs through a combination of electric double-layer capacitance and oxidation/reduction pseudocapacitance based on a new class of materials.
Supercapacitors have the ability to store unusually large amounts of charge compared to comparably-sized electrolytic capacitors. For example, a supercapacitor having the same dimensions as a D-cell battery is capable of storing hundreds of farads (F) of charge. In contrast, an electrolytic capacitor having the same dimensions will typically store a few tens of millifarads (mF) of charge. Thus, supercapacitors hold promise for storing electrical energy at high power densities and with high charge and discharge rates for a variety of applications including hybrid and/or electrical automobiles, industrial equipment (e.g., rubber tire gantries), electrical grid load-leveling, and power tools.
In comparison to batteries, supercapacitors currently in production have energy densities in the range of only about 1 to 10 Wh/kg in contrast to secondary cell batteries which have energy densities of 10 to 100 W hours per kilogram. On the other hand, the power density (being a measure of how quickly the energy may be released) for a supercapacitor is 10 times higher than that of a secondary cell battery or about 1000-5000 W per kilogram.
Batteries employ a Faradaic energy storage mechanism employing a chemical change in oxidation state of the electroactive material via electron transfer at the atomic or molecular level. This mechanism is relatively slow, which limits the power density of the batteries, and further creates stresses that limit cycle-life.
Conventional supercapacitors employ a non-Faradaic mechanism in which energy is stored electrostatically at the interface between electroactive solid and liquid electrolyte. In this case, there is no change in oxidation state (i.e., no electron transfer at atomic or molecular level). Compared to Faradic processes, non-Faradic processes are very fast, which allows for high power density, and they create little stress in the electroactive materials, which improves cycle-life.
The high capacitance of a conventional supercapacitor is obtained by the creation of an electric double layer at the electrode/electrolyte interface in which charges are separated by a distance of a few nanometers. Conventional supercapacitors are also known as electric double-layer capacitors.
Improved supercapacitors have been developed that store charge by a combination of Faradaic and non-Faradaic mechanisms. Supercapacitors using a combination of Faradaic and non-Faradic mechanisms will be termed herein “pseudocapacitors”. Generally, a pseudocapacitor uses a metal oxide having relatively high electrical conductivity as an electroactive material (e.g., RuO2), or else an electroactive polymer.
Pseudocapacitor electrode architectures based on these metal oxides generally use some carbon and/or organic binder in the fabrication process. The carbon improves electron transfer between the active material and current collecting plate. The binders hold the metal oxide and carbon particles together forming a continuum.
Pseudocapacitors can have both high energy density and power density; however, electroactive materials that perform well (e.g. RuO2) can be expensive, scarce, and toxic, limiting their application and attractiveness.